Rheological insights into vat photopolymerization 3D printing of colloidal suspensions

ABSTRACT This study investigates how shear thinning and thixotropic recovery govern the vat photopolymerization (VPP) processing of high-viscosity fumed silica (FS)-filled colloidal photopolymer resins. Hydrophobic FS (Aerosil R972) was incorporated into a commercial acrylate resin to create suspensions with tunable viscosity, yield stress, and recovery behavior. Rheological measurements showed that FS loading induces strong non-Newtonian behavior, with reversible network breakdown under shear and time-dependent structural recovery after shear cessation. Photo-rheology indicated that FS loading up to 10 wt% did not significantly alter bulk curing kinetics under the tested conditions, while ultra small angle X-ray scattering (USAXS) confirmed post-shear particle re-agglomeration and network rebuilding. Printing experiments demonstrated that shear thinning enables processing of the 4 wt% FS formulation despite its zero-shear viscosity exceeding the commonly cited VPP limits. However, the 8 wt% FS resin exhibited defects, including bottom-layer overgrowth, layer thinning, and slanting, highlighting the narrow processing window for highly filled thixotropic systems. Mechanical testing of green-state specimens revealed a slight tensile reinforcement from R972 FS. Interlayer wait time showed a stronger influence on layer morphology, dimensional accuracy, and mechanical properties than build-head speed within the tested conditions and samples, where a wait time of 60 seconds provides necessary thixotropic recovery at the cost of ultimate mechanical strength. While SEM of the printed samples showed subtle differences in morphology under different wait times, the study revealed that more complex flow and recovery processes exist beyond the rheological preview. Overall, this work establishes how a rheology-guided framework can support our understanding and processing of high-viscosity colloidal resins in VPP.